Sound-reproducing apparatus



J1me 7 1927;

v c. w. RICE scum) REPRODUCING APPARATUS Filed March 27, 1924 2 Sheets-Sheet 1 I I I a I Inventor:

1 ChQSteF'W.RiQQ,

H is Attorney.

June 7 1927. 1,631,646

. c. w. RICE SOUND REPRODUCING APPARATUS Filed March 27, 1924' 2 Sheets-Sheet 2 H85. I Fig.6.

Fiigfl'.

Q 7 Inven tor- Chesterwfiice, y W

His Attorney,

projected from Patented June 7, 1927.

UNITED STATES PATENT OFFICE.

CHESTER w. RICE, 0F SCHENECTADY, NEW YORK, AssrGNOR T0 GENERAL ELECTR c COMPANY, A CORPORATION OF NEW YORK.

SOUND-REPRODUGING APPARATUS.

Application filed March 27, 1924. Serial No. 702,452.

My present invention relates to apparatus for producing sound from electric currents corresponding to original sounds, and more particularly to devices now commonly known as loud speakers.

One of the objects of my invention is to provide a device of the class mentioned whereby the reproduction of sounds may be made with greater accuracy and fidelity than with devices of this class previously-cmployed.

A further object of my invention is to provide an apparatus in which the usual horn may be dispensed with and the sound the device in a more satisfactory manner than with the usual form of horn.

The novel features which I believe to be characteristic of my invention are pointed out with particularity in the appended claims. My invention itself. however, both as to its organization and method of operation will best he understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a cross sectional view of a device constructed in accordance with my invention, Fig. 2 is an elevation of one end of the sound reproducing device and Figs. 3 to 10 inclusive are diagrammatic views of further modifications of my invention.

The device illustrated in Figs. 1 and 2 compris es a diaphragm 1 of conical formwhich is supported in an annular air gap'for'n'ied by the concentric pole pieces 2 and 3. T hose may be the pole pieces of a permanent magnet or the magnetic field may be produced by means of an exciting coil 4 surrounding fported on pole piece the pole piece 2. The diaphragm 1 is provided. with a flange 5 at its base evtendiruc at right angles to its axis whereby it is sup 3 by means of a clamping ring 6. The apex of the diaphragm may also be supported. in that case the apex of the cone is flattened and the flattened portion secured at the center to the pole piece 3 by means of a bolt 7 extending through the pole piece 3.

The diaphragm '1 instead of being rigid as in most forms of loud speakers may be made extremely flexible. It may consist of a coil 8 to which the actuating currents are material 9 such as cloth or rubber. One

method which I have employed-successfully in constructing the diaphragm consists in painting a metal form with a vulcanizable rubber cement, winding the coil Over this cement, painting another layer of the cement over the coil, and then baking the whole to vulcanize the cement.

, The diaphragm 1 being entirely non-magnetic, there is no tendency for it tomove toward either pole piece in the absence of current in the coil 8. Therefore the elastic restoring force required to return it to its mean position between the pole pieces is very small and 'a fundamental resonance point of the diaphragm may beseeured at a frequency below any of the important voice frequencies. The driving force is also applied uniformly'over substantially the entire working area of the diaphragm so that the tendency of the diaphragm is to vibrate as a whole without breaking up into higher modes of oscillation. These are essential qualities for a diaphragm for producing uniform sound reproduction over the entire working frequency range.

- Pole piece 3 is provided with a plurality of radial slots 10. to permit the escape of the sound produced.

Another feature which is essel'itial to the satisfactory operation of a loud speaker is the avoidance of resonant air chambers next to the diaphragm. For example, if the space behind the diaphragm is completely enclosed there will: be a certain frequency at which the enclosed air will furnish an elastic restoring force to the diaphragm causing a strong resonance effort, and other frequencies at which the air will so resist the motion of the diaphragm as to greatly reduce its amplitude of vibration. In the construction illustrated, by reasonof the long narrow air gap the space behind the diaphragm is enclosed except at the outer edge and hence the air cannot readily escape. The tendency of this construction to introduce resonant air effects may be eliminated by providing radial slots 11 in the pole piece 2 for the purpose of adequately venting the air spacebehind the diaphragm.

Let us now consider theproblem of Obtaining a true and useful sound reproduction from the to and fro vibrations of a small diaphragm.

If the diameter of the diaphragm is small compared'with )t/2-rr it can be shown that the power radiated from thediaphragm in the from of sound waves is givenby where #:normal density pf fluid grams cm. w:21rf where f=frequency of vibration in cycles per second.

sztotal area of diaphragm (both sides) sq. cm. 7

m max. amplitude of vibration cm.

a velocity of sound in fluid*(cm. per sec.).

In the above the circular disc is assumed.

to be vibrating to and fro in free space. If we now assume that only one side of the diaphragm is allowed to act on the air as by enclosing the back, the sound radiation is thereby greatl increased because the air cannot circulate etween the front and back of the diaphragm.

Under these conditions the sound radiated becomes ,where 8 is the area of the exposed side of the diaphragm, If we now take the ratio of (2) to (1) we have P (back enclosed) 271r6 8 P (both sides open)* 8:0 .9

ergs per sec.. .--(2) As an example consider the followingcasez.

] c 3.3 a; 10 cm. per 'sec. (air). v p

s 25 sq. cm. total area of both sides; 8 12.5 sq, cm, exposed areaof diaphragm. v a I For W w: 5000 Ratio: 4.60" v ,l V w 500 Ratio 460. i From this wefjsee that the low tones are almost entirely "lost when both-sides of the diaphragm are exposed, and even the'high tones are considerably. weaker. The reason for the loss of is because the air can circulate between the front and the back of the diaphragm with out undergoin any a preciable compression. In other wor s', the .laphragm is short ci r-i cuited. If instead of closing in the back of the diaphragm, we lengthen-the path to a distance of about a wavelength or more, by

. which air can passfrom the front to the back of the diaphragm,there is no lon er a'v short circuit and good radiation may esecured.

" This'efli'e'ct may be had'by surroundingthe diaphragm by a battle 13' of fairly massive or rigid material in such a manner as to efi'ec- 'tively prevent circulation. Under these con-.

ditions, the sound radiated from each side of the diaphragm is given by 4 2 P=p ggf ergs per sec (4) where s, area of the side of diaphragm under consideration and B. solid angle into which'the radiation flows 2 11' for hemisphere, etc.

From this we see that the use of a battle not only eliminates the difliculties met with in providing a properly damped chamber back of the diaphragm, but increases the sound output. Here one side of the diaphragm radiates into a solid angle of 2 units, or a hemisphere as compared with 4 units when used with an enclosed back and without the baffle and,

therefore, radiates twice as much power, and

secondly, both sides of the diaphragm give useful radiation so that there is in all four times as much sound energy radiated with a large bafile, as with no baflie and the back of the diaphragm enclosed. From the above we also see that a baflie may be used to advantage in increasing the sound radi-.

.but at the same time lowers the general itch level of the sound emitted andintro uces disturbing resonant and anti-resonant effects from-which a baffle is substantially free.

With a battle, the general pitch level of I the loud speaker can becontrolled to a considerable extent by the size of the baiile, a lar e baflie giving a lower pitch. I have foun that a bafiie having an'eflective diameter of from two to three feet produces 7 satisfactory results. It is generally accepted that in order to obtain the bestspeech reproduction the reproducing device must cover the low tones effectivelya range of frequency extending,

from 100 cycles to 4,000 cycles. The length of asound waveof 100 cycles is approximately eleven feet. lVith-a baiile three feet in diameter, therefore, the path between the front andback of the diaphragm for the lowest important voice frequency is approxi- ,mately aquarter wave length." The effect described may obviously be obtained toffa" large degree solely by the use of Ja dia-- phragm so large that the path forsound' waves between-opposite sides of the central portion of'the diaphragIm .is of the order of- 'magnitude mentioned. portion ofthediaphragm is relatively inn this case the outer" Ill efficient as a radiator but may act as an effective battle for the active inner portions of the diaphragm.

I have also found that the baffle may be given a conical shape as indicated in Figs. 3 and 4: provided the angle is not much less than 90, without bringing in the detrimental resonant efl'ects characteristic of horns. Another effective method is to make the battle in the form of a shallow'box with open back as indicated in Figs. 5 and 6. Naturally many other arrangements will suggest themselves which. are equally effective and non-resonant, such, for example, as a pyramid shaped bafile, as shown in Figs. 7 and 8.

I have described my invention with particular reference to its use with a diaphragm having so small elastic restoring force that the fundamental resonance point is at a frequeney below any of the important voice frequencies. Such a diaphragm I term an inertia controlled diaphragm. The volume of sound produced by such a diaphragm may be made to be substantially independent of the frequency provided the current supplied for actuating the diaphragm is independent of the frequency for equal sound power input at the various frequencies. A current of the desired characteristic may be obtained from a pick-up device of the usual type which has an elastic controlled diaphragm that is one in which the fundamental frequency is above the important voice frequencies.

My invention, however, is not limited in its utility to the use with this particular type of diaphragm as some of the advantages which I have pointed out may be obtained with other types of diaphra s. I

What I claim as new and esire to secure by Letters Patent of the United States, is

1. The combination in a sound reproduc- "ing apparatus-of a diaphragm, actuating means for said diaphragm and a bafiie surrounding saiddiaphragm, said baffle having .such configuration as to materially decrease the solid angle into which energy will be radiated from said diaphragm without at the same time introducing resonant efi'ects which would modify materially the sound reproduction;

2. The combination in a sound reproducing apparatus of-a vdia hragm so arranged that air can circulate reely on both sides thereof, and a bafiie surrounding said diaphragm for increasing the length of the path through-the air between the two sides of the diaphragm to a distance at least approximately equal to one ,quarter wave length of the longest useful'sound waves to be'reproduced thereby. y

3. The combination in a sound reproduc ing apparatus of a diaphragm whichis so constructed and supported that its funda- -baitle having such configuration as to mamental natural frequency of vibration is below the lowest important voice frequency, actuating means for said diaphragm and a baffle surrounding said diaphragm, said terially decrease the solid angle into which energy will be radiated from said diaphragm without at the same time intro-' ducing resonant etl'ects whichwould modify materially the sound reproduction.

4. The combination in a sound reproducing apparatus of a diaphragm so arranged that air can circulate freely on both sides thereof, and a battle surrounding said diaphragm and of such dimensions that the path through the air between the two sides of the diaphragm has 'a length of at least the order of magnitude of one quarter wave length of the longest useful sound waves to be reproduced thereby.

5.- The combination in a sound reproducing apparatus of a diaphragm so arranged that air can circulate freely on both sides thereof, and a battle surrounding said. diaphragm for increasing the length of the path through the air between the two sides of the diaphragm, said-battle having such configuration'as to materially decrease the solid angle into which energy will be radiated from either side of' the diaphragm without at the same time introducing resonant effects which would modify materially the sound reproduction.

6. The combination in asound reproducing apparatus of a diaphragm which is so constructed and supported that its fundamental natural frequency of vibration is be- 'low the lowest important voice frequency,

actuating means for said diaphragm and a bafiie surrounding said diaphragm for increasing the length of the path through the air between the two sides of the diaphragm to a distance at least approximately equal to one quarter wave length of the longest useful sound waves to be reproduced thereby.-

7.'The combination in a sound reproduc- .ing apparatus of a diaphragm which is so constructed and supported that its fundamental natural frequency of vibration is below the lowest important voice frequency,

said diaphragm being so arranged that air can circulate freely on both sides thereof, actuating meansfor said diaphragmand a baflie surrounding said diaphragm, said baf: fie having such configuration as to materially decrease the solid angle-into which energy will be radiated from either side of said diaphragm without at the same time introducing resonant effects -which would modify materially ,thejsound reproduction.

8. The combination; in a sound reproducing apparatus ofh'diaphragmiwhich is so constructed and supported that its fundamental natural frequency of vibration is below the lowest important voice frequency said diaphragm being so arranged that air can circulate freely on both sides thereof,

5 actuating means for said diaphragm and a baiile surrounding said diaphragm, and of such area that the path through the air between the two sidcsof the diaphragm has a length of at least the order of magnitude of a quarter wave ful sound waves to be reproduced thereby.

9. The Combination with a conical diaphragm of a magnetic pole piece having an opening of conical form in its end to receive said diaphragm, a second magnetic pole piece of conical form which is surrounded by said diaphragm, the two pole pieces being so related that the diaphragm is supported in a long narrow air gap between the pole pieces and a plurality of openings through both of said pole pieces to permit of the ready escape of air from said air a b ibyThe combination with a conical diaphragm of a magnetic pole piece having an opening of conical form in its end to receive said diaphragm, a second magnetic pole piece of conical form which is surrounded by said diaphragm, the two pole pieces being so related that the diaphragm is supported in a long narrow air gap between the pole pieces and a plurality of radial openings through both of said pole pieces to permit of the ready escape of air from said air gap.

r11. The combination in a comical'tdiaphragn' of a magnetic. pole piece having an opening of conical form 'in its end to receive said diaphragm, a second magnetic pole 40 piece of conical form which is surrounded by said diaphragm, the two pole pieces being so relatedthat the diaphragm is supported in a long narrow air gap between the pole pieces, openings in said second pole piece for permit-ting the escape of air from said air gapon outside said diaphragm, and a plurality 'of radial slots in said first pole piece to permit of the escape of air from the other side of said diaphragm.

.12. A diaphragm for sound reproduction comprising a sheet of rubber of conical form in which an actuating coil is embedded, said diaphragn'i having at. its base a flange formed by an-extensiou of the rub- I her sheet. said flange extending at a right angle to the axis of the diaphragm.

13. A diaphragm for sound reproduction comprising a sheet-of rubber of conical form in which an actuating coil is embedded, said 50 dia phragm having at its base a flange formed length of the longest usehaving its apex flattened and means for supporting said diaphragm from its base and its apex.

14. The combination in a sound reproducing apparatus of a cabinet, a diaphragm which is mounted in an opening in the front of said cabinet in such a way that air can circulate freely on both sides thereof, said cabinet having other openings to prevent the introduction of resonant air cavities at the back of the diaphragm and being of such dimensions that the length of the path through the air between the two sides of the diaphragm is of the order of magnitude of at least a quarter wave length of the longest useful sound waves to be reproduced bythe diaphragm. 1

15. The combination in a sound reproducing apparatus of a cabinet, 'a diaphragm which is mounted in an opening in the front of said cabinet in such a Way that air can circulate freely on both sides thereof, said diaphragm being so constructed and supported that its fundamental natural frequency of vibration is below the lowest important voice frequencies, said cabinet having other openings to prevent the introduction of resonant air cavities at the back of the diaphragm and being of such dimensions that the length of the path through the air between the two sides of the diaphragm is of the order of magnitude of at least a quarter wave length of the longest useful sound waves to be reproduced by the diaphragm.

16. The combination in a sound reproducing apparatus of a diaphragm mounted in a fluid medium and so arranged that the fluid medium in which it is mounted can circulate freely on both sides thereof and a baffle surrounding said diaphragm, said bafile being of such dimensions that the path through the fluid medium between the two sides of the diaphragm has a. length of at least the order of magnitude of a quarter wave length in that medium of the longest useful sound waves to be reproduced thereby.

17. The combination in a sound reproducing apparatus of a diaphragm so constructed and arranged that air can circulate freely on both sides thereof and that the length of the path through the air between the two sides of the effective sound radiating portions v thereof is at least approximately equal to a quarter wave length of the longest useful 4 sound waves to be reproduced thereby.

In witness whereof, I have hereunto set my hand this 26th day of March, 1924.

' CHESTER W. RICE. 

